The present invention relates to methods of optical to electronic conversion, in particular to photo-detection with devices suitable for integration with Complementary Metal Oxide Semiconductor (CMOS) technology.
The devices and fabrication technologies used for light-sensing are highly dependent on the frequency or wavelength of the light to be sensed. Usually, for imaging purposes, the spectrum is divided into:                Ultra-Violet (UV), wavelengths shorter than 0.4 μm        Visible, wavelengths in the range of 0.4 μm to 0.7 μm        Short-Wave Infra-Red (SWIR), wavelengths in the range of 1 μm to 3 μm        Mid-Wave Infra-Red (MWIR), wavelengths in the range of 3 μm to 5 μm        Long-Wave Infra-Red (LWIR), wavelengths in the range of 8 μm to 12 μm        Very Long-Wave Infra-Red (VLWIR), wavelengths longer than 20 μm        
For wavelengths in the visible range, a wide range of devices and materials can be used, with the most common types being silicon pn-junction photo-diodes, either in CCDs or CMOS image sensors. For UV detection, it is usually desirable to have “solar-blind” detection, i.e., absorption of UV without absorption of the visible wavelengths. This can be achieved with pn-junction or heterojunction photo-diodes made with materials whose bandgap energy is larger than the energy of the photons with wavelengths in the visible range. Examples of such materials are Silicon-Carbide (SiC), GaN, AlGaN, AlN, ZnO, etc. It is also possible to have solar-blind detection with thinned CCDs. These devices make use of the fact that the coefficient of absorption of most materials, including silicon, increases for shorter wavelengths. Therefore, solar-blind UV detection is achieved by reducing the thickness of the light sensing material, so that the signal generated by visible light is much weaker that the signal generated by UV light. Yet another possibility for solar-blind UV detection is to place a visible-blocking filter in front of the light-sensing elements.
The sensing of the different IR bands of wavelengths can be approached with very different devices, operating under different physical mechanisms, typically with very different materials systems.
The SWIR band includes the 1.310 μm and the 1.550 μm ranges of wavelengths that are used for fiber-optics communications. These wavelengths can be absorbed by pn-junction or heterojunction photo-diodes made with Germanium (Ge) or Indium-Phosphide (InP). It has been shown that SiGe and/or SiGeC random alloys and/or superlattices can also detect these signals.
The MWIR band can be approached by devices operating on band-to-band transitions, such as pn-junction and heterojunction photodiodes, provided that the materials used have a small enough bandgap, for example HgCdTe. Other devices, such as Quantum Well Infrared Photodetectors (QWIPs) can also be used. QWIP devices operate with intersubband transitions, i.e., transitions between discrete energy levels either in the conduction-band or in the valence-band of quantum wells. Some common materials employed are GaAs—AlGaAs, InGaAs—InAlAs, InGaAs—InP, GaAs—GaInP, GaAs—AlInP, etc. Another physical mechanism employed for the detection of these wavelengths is the Heterojunction Internal Photoemission (HIP). This mechanism can be implemented with semiconductor-semiconductor heterojunctions or with metal-semiconductor heterojunctions (also known as Schottky-diodes). The barrier height at the heterojunction determines the longest wavelength possible to detect. QWIPs and HIP devices are unipolar and can be designed to operate with either electrons or holes. The performance of all these different types of devices can be significantly increased by lowering the temperature of operation. Typical temperatures for the HIP devices are around 77K (liquid nitrogen cooling).
The LWIR band can be sensed with essentially the same devices used for the MWIR, but with the appropriate fine-tuning of the device-parameters. For devices based on band-to-band transitions this requires a decrease in band gap energy, for QWIP devices this requires the energy difference between the quantized levels to be decreased, that is, the width of the quantum well to be increased, and for HIP devices the heterojunction or Schottky “barrier height” needs to be decreased. All these result increased noise and decreased performance of the light-detection, which can be counter-acted by further decreasing the operating temperature.
The VLWIR band can be covered by some of the devices mentioned for LWIR, including HIP devices, but all need to operate at very low temperatures of around 4K (liquid helium cooling). In addition, there is another physical mechanism that is used for these wavelengths: excitation of doping impurities. The energy levels of doping elements are usually very close to either the conduction band or the valence band of the host semiconductor (Si, Ge, GaAs, etc.). At room temperature the doping impurities can be taken to be fully ionized. The energy difference between in the “band of impurities” and the host semiconductor corresponds to the energy of photons in this band. At very low temperature, the impurities remain neutral and the host semiconductor remains “intrinsic”. Therefore, photons can excite the impurities and inject carries into the energy bands of the host semiconductor, thereby producing a detectable signal.
Multispectral and hyperspectral imaging, covering at least some of the spectral bands mentioned above, offer many interesting possibilities for the retrieval, processing and subsequent display, of data that the human eyes cannot perceive, and that can have a wide range of applications. It has been very challenging to fabricate monolithically integrated sensors for the different wavelength bands (multi-spectral sensors), having pixel density and cost of fabrication comparable with the state of the art commercial CCDs and CMOS image sensors. The main reason for this situation is the lack of suitable device/materials and process architectures capable of fabricating sensors covering most of the spectral ranges mentioned above.
Image sensing in different ranges of wavelengths requires different types of photo-detectors. The differences between photo-detectors range from materials system, device physics, device architecture, mode of operation, etc. The reason behind this diversity in photo-detection technologies is due to, firstly, all the possible different approaches to tackle the physics involved in the absorption of photons of a particular wavelength, and secondly, what are the best technologies for each range of wavelengths of interest.
Conventional photo-detectors covering the wavelengths in the visible range, such as Charge Coupled Devices (CCDs), CMOS photo-diodes and CMOS photo-gates are made on silicon substrates and involve what are called “band to band transitions” in which electron-hole pairs are generated. For this type of photo-detection a photon is absorbed by scattering one electron from the valence band to the conduction band, thus creating a “hole” in the valence band.
There is strong interest in “solar-blind” or “visible-blind” UV photo-detection for which photo-detectors made with wide band-gap materials such as SiC (Silicon Carbide) or GaN (Gallium Nitride) have been demonstrated. Such materials have band-gaps larger than the energy of the photons in the visible range, and therefore band-to-band transitions cannot take place. Photons in the UV range of the spectrum have energy larger than the band-gap of these materials and can therefore generate electron-hole pairs through band-to-band transitions.
In the Infra-Red (IR) portion of the spectrum, the situation is little bit more complicated, as there are several distinct regions of the spectrum. Different technologies are under development to improve the performance for each of these wavelength intervals. For example, Silicon devices such as CCDs, pn-junction diodes or p-i-n (PIN) junction diodes, are capable of absorption in the SWIR band (0.7 μm<λ<1 μm) and germanium devices can absorb wavelengths up to 1.6 μm. For the MWIR range (3 μm <λ<5 μm), Heterojunction Internal Photoemission (HIP) detectors making use of Schottky-junctions between p+-Silicon and PtSi (Platinum Silicide) seem to offer good performance. On the other hand the best performing devices and materials for LWIR (8 μm<λ<12 μm) seem to be HgCdTe (Mercury Cadmium Telluride—MCT) photodiodes. For LWIR SiGe/Si HIP devices have also been fabricated but their performance seems considerably lower than that of MCT-based devices. As a result, sensors made with MCT can operate without cooling while the best SiGe/Si HIP devices require cooling to 77K.
The fact that each range of wavelength requires a different type of photo-detector and/or a different materials system, has made impossible the fabrication of 1D and/or 2D arrays (or focal planes) of “color” pixels capable of capturing multiple-wavelengths in the visible and invisible parts of the spectrum, such as UV, Visible, SWIR, MWIR, LWIR.
Conventional solutions include the sensing with separate cameras each with its own set of lenses, focal planes, along with all the required circuitry to produce still images and/or video signals. Hybrid schemes have been developed, in which there is a single set of lenses, but still require the splitting of the incoming light through prisms and wavelength filters onto different focal planes with the type of photo-detectors suitable for each desired range of wavelengths.
Having a single set of lenses and a single focal plane capable of image sensing in all the desired wavelengths would present an extraordinary advancement towards the goal of “sensor fusion”. Therefore, it is of extreme relevance to develop a process flow for the fabrication of photo-detector devices suitable for monolithic integration with CMOS circuitry for 1D and/or 2D arrays of Passive-Pixel Sensors (PPS), and/or 1D and/or 2D arrays of Active-Pixel Sensors (APS), in which each pixel has multi-wavelength (visible and invisible) sensing capability. The present invention shows how to achieve this goal.